1. Field of the Invention
The invention pertains to the field of small optical display systems. More particularly, the invention pertains to apparatus and methods for enhancement of a real image projection system through the use of several combinations of methods of aberration reduction and system brightness enhancement. The primary enhancement is reduced ghosting and reduction of astigmatism, common with small real image systems wherein the viewing distance is relatively close.
2. Description of Related Art
The present invention pertains to a real image projection system, and in particular, to a system in which an image of a real object is formed in space, giving the illusion that a real object exists at that point in space, when in reality it does not. Real image projection systems normally incorporate spherical or parabolic mirrors for imaging. In large systems, where the viewer is located at a significant distance from the image being viewed, optical aberrations, such as, for example, spherical aberrations, and astigmatism in particular, are not as much of a problem as in smaller systems, where the viewer is located close to the image. Astigmatism causes eye strain when viewing the image for long periods of time, and this has been one of the primary reasons that small real image projection systems have not been widely incorporated in gaming applications, as well as in workstation applications.
Another reason for the lack of wide-spread acceptance of small real imaging systems is that ghost images in the systems are much more noticeable, when viewing the display from a close distance. Many approaches have been used to reduce ghosting, including tinted beamsplitters and polarizers, none of which are extremely effective. Even with the use of a circular polarizing window, the ghost images are visible because of elliptical polarizing effects of the 45 degree beamsplitter, although they can be significantly reduced. The circular polarizing windows typically have a maximum transmission of 42%, and this significantly reduces image brightness. Thus, in an arcade or other public area that is brightly lit, the real image usually is difficult to see.
Other optical aberrations present problems for real image projection systems. For example, field curvature distortion is a significant problem for smaller systems, because of the shorter focal lengths typically associated with small systems. For example, a rectangular shape displayed on a CRT screen projects as a “fish-eyed” real image of the target object. The sides of the rectangular image appear to bow outward and the center of the rectangular image appears magnified, as compared to the edges. This is a natural phenomenon of spherical mirrors, and cannot normally be corrected without a significant number of additional lenses in the beam-path, which makes the display system significantly larger in physical size, as well as making the cost of manufacturing such displays prohibitive.
Tilting of the curved mirror has been attempted in other prior art real imaging systems, but this has primarily been accomplished with a beamsplitter to divert the converging imaging beampath at a 90 degree angle, in relation to the diverging target beampath. This method has not been widely accepted because of the additional focal length distance required to form the image at the viewing position, which significantly reduces the field of view. For example, in a non-tilted system having a 48″ radius mirror, tilting of the curved optic at a 15 degree angle to eliminate ghosting and form the image at the same point in space would require a 54″ radius mirror, thus reducing the field of view by a substantial amount. In textbooks, a spherical mirror is used to display a real image of a flower vase in a system similar to the tilted mirror system of the present invention, however, the described prior art system would not allow the use of a beamsplitter for providing a secondary virtual background image. The present invention utilizes a unique configuration that overcomes this problem of the prior art, with minimal loss of system brightness transmission, and without reducing fields of view.
The biggest problem with a tilted system utilizing a beamsplitter is that system transmission is only approximately 15%. This is a result of the imaging beam both transmitting then reflecting from the beamsplitter, which reduces transmission by 50%.
Tilting of the optic also causes serious aberrations to the image, because the beampath strikes the tilted (i.e., 15 degree tilt) curved mirror at 15 degrees to the mirror axis. The beampath strikes the spherical mirror at 15 degrees, which is actually an elliptical curve at the plane of intersection. The spherical surface of revolution becomes more elliptical as the angle approaches the outer top edge of the spherical mirror.
Optics have been designed to compensate for some of these aberrations, such as, for example, spherical aberrations, through use of the Mangin mirror. This is a mirror that has a reflective convex spherical surface of longer spherical radius, and a transmissive concave spherical surface of shorter radius. However, this approach is not practical for a real image projection system, because the image source or target is not a point at the focal point or center of curvature of the mirror, as in a single point imaging system. In a real image projection system, the target usually is a rectangle, such as a monitor screen, where only the center of the screen is on the axis or at the focal point of the mirror. The Mangin dual curve corrective mirror could be significantly improved by replacing the concave spherical surface with an aspheric surface of revolution, which will reduce the astigmatism for points offset from the axis of the mirror. Thus, a Mangin mirror incorporating two spherical curves is extremely effective for points along the axis of an on-axis system, but the problem of astigmatism becomes progressively worse as the target point deviates from the axis of the mirror curvature. An aspheric curve on the concave surface would optimize the correction and reduce the astigmatism for a larger area around the axis or focal point.
One other reason that small systems have not become mainstream is because of the difficulty in producing the curved optics in reasonable volume. The problem is compounded when corrective optical curvatures are incorporated.